Direct reduction plants for producing direct reduced iron, known as DRI (sponge iron) or hot briquetted iron (prereduced materials useful as feedstocks for iron and steelmaking), currently produce such products by contacting a reducing gas, composed principally of hydrogen and carbon monoxide, at temperatures in the range from about 750.degree. C. to about 1050.degree. C., with a bed of particulate iron-containing material in the form of lumps or pellets. Examples of such processes are described in U.S. Pat. Nos. 3,749,386; 3,764,123; 3,816,101 4,336,063; 4,428,072; 4,556,417; 5,078,787; 4,046,557; 4,002,422 and 4,375,983.
It is known from U.S. Pat. No. 5,238,487 to Hauk et al. to produce direct reduced iron in a reduction reactor fed with top gas effluent from a first reaction reactor attached to a melter-gasifier. The invention of the '487 patent addresses the problems which arise when a reducing gas having a high content of carbon monoxide is heated in a heat exchanger type direct fired heater due to carburization and metal dusting of the heater pipes. More particularly, the '487 patent teaches to withdraw the oxidants in the top gas effluent from the first reactor and to utilize this modified gas in a second reduction reactor comprised in a recycle loop of the reducing gas. The recycle loop comprises the reactor; a cooler/scrubber; a CO.sub.2 removal unit; a gas heater to heat the gas to a temperature between 200.degree. C. and 500.degree. C.; and a partial combustion chamber to heat the gas to a temperature between 750.degree. C. to 850.degree. C., required to carry out the reduction reactions. Alternative embodiments of the invention comprise addition of inhibitors, for example: SO.sub.2, H.sub.2 S, NH.sub.3, (CN).sub.2, NO.sub.2 or Cl.sub.2, individually or in combination, to the reducing gas prior to be heated in heater thus making it possible to increase the temperature of the gas in the heater and decreasing the consumption of oxygen in the partial combustion stage.
It is also known from U.S. Pat. No. 3,909,244 to Rose et al. to modify the composition of a reducing gas produced in a natural gas-stream reformer so as to obtain a higher content of hydrogen in the reducing gas. This patent teaches that reducing iron ores with a gas mainly composed of hydrogen increases the rate of reduction and increases the economy of the regeneration of the recirculating gases. There is no teaching or suggestion in this patent as to the utilization of a reducing gas effluent from a first reduction reactor or solving the problems posed by iron ores having a high swelling index when reduced with carbon monoxide.
U.S. Pat. No. 4,363,654 to Frederick et al. is directed to a process for producing a reducing gas for a direct reduction or blast furnace which comprises partially oxidizing oil and/or coal in the presence of air to produce a gas stream containing hydrogen and nitrogen together with other gases, treating the gas stream to remove essentially all gases other than hydrogen and utilizing said hydrogen for reducing iron ores. This patent however does not teach or suggest to have a combination of a melter-gasifier and a first reduction reactor and a shifter for modifying the reducing gas composition to be utilized in a second reduction reactor. No suggestion is made in this patent about the problems encountered in reducing iron ores with a high swelling index when reacted with carbon monoxide.